The invention relates to pressure-sensitive adhesive (PSA) tapes and to a method of producing pressure-sensitive adhesives (PSAs) for the adhesive bonding of printing plates, the PSA possessing a very low peel increase on polar surfaces and being easy to reposition, and, after the bonding on the printing cylinder, exhibiting low edge lifting of the printing plate and of the assembly formed from adhesive plate-mounting tape and printing plate.
The printing industry knows of a variety of techniques for transferring designs to paper, for example, by means of print originals. One possibility is that known as flexographic printing. One embodiment of flexographic printing, in turn, is the use of multilayer photopolymer printing plates with a flexible substructure, this type of printing having been part of the prior art for a relatively long time. The printing plates in this case are composed of a plurality of layers of different polymeric materials each with specific functions. For example, the “nyloflex MA” and “nyloflex ME” printing plates from BASF AG have three layers, namely a light-sensitive relief layer, a stabilizing film below it, and an elastic carrier layer.
In the flexographic printing process, flexible printing plates are bonded adhesively to printing cylinders. This bonding is generally carried out using double-sided PSA tapes, on which very stringent requirements are imposed. For the printing process, the PSA tape is required to have a certain hardness but also a certain elasticity. These properties must be set very precisely in order that the printed image produced yields the desired result in accordance with the requirements. Further stringent requirements are imposed on the PSA itself, where the bond strength must likewise be high so that the printing plate does not detach from the double-sided PSA tape or the PSA tape from the cylinder. This must be so even at increased temperatures of—40-60° C. and at high printing speeds. In addition to this property, however, the adhesion properties of the PSA must also be reversible, since it is often necessary to bond the printing plates and then to detach them again for repositioning. This detachability should also exist in the case of an adhesive bond over a prolonged time period (up to six months). Moreover, it is desired that the PSA tape and especially the printing plate can be removed again without destruction thereof, i.e., without great application of force. In addition, no residues should remain on the printing plate or on the cylinder. In summary, then, very stringent requirements are imposed on the double-sided PSA tapes that are suitable for this utility.
U.S. Pat. No. 4,380,956 describes a process for mounting a printing plate for the flexographic printing process. PSAs are among the adhesives used for this process, but have not been specified in any greater detail.
GB 1,533,431 claims a double-sided PSA tape including an elastomeric layer which in turn is foamed by fragile air bubbles. The air bubbles are destroyed under pressure during the flexographic printing application.
U.S. Pat. No. 4,574,697 claims double-sided PSA tapes comprising as their carrier material a flexible polyurethane foam affixed to a PET (polyethylene terephthalate) film. The outer layers are composed of pressure-sensitive adhesives. The PSA tape described is to be reversible and to be removable from the printing cylinder and from the printing plate. A similar product structure has been described in EP 0 206 760. There, the flexible foam carrier used was a polyethylene foam.
U.S. Pat. No. 4,574,712, in analogy to U.S. Pat. No. 4,574,697, describes a similar PSA tape construction. Here there is a restriction on the PSAs, namely that the bond strength to the printing plate and to the printing cylinder should be lower than to the carrier film and the carrier foam.
U.S. Pat. No. 3,983,287 describes a laminate whose carrier material comprises an incompressible elastomer. Compressibility is achieved by means of beads which are destroyed under pressure and which therefore produce flexibility.
U.S. Pat. No. 5,613,942 describes PSA tapes which are especially suitable for bonds on wet surfaces. It is mentioned, inter alia, that such tapes are suitable for bonding printing plates.
U.S. Pat. No. 5,476,712 likewise describes a double-sided PSA tape which is used in the flexographic printing process. This PSA tape contains, in turn, a thermoplastic elastomer, the structure present in this case being a cellular structure produced by means of expanding microparticles.
In the cases mentioned above, a very large number of different PSAs are used. Natural rubber adhesives possess good tack properties but lack great shear strength at room temperature, and age as a result of degradation via the double bonds present in the polymer.
SIS-based or SEBS-based PSAs are generally very soft and tacky and tend to soften at high temperatures as well. If the printing plate is bonded to the printing cylinder under tension using an SIS or SEBS PSA, the printing plate tends to detach, despite the fact that the bond strength is high.
Acrylate PSAs, on the other hand, are very suitable for bonding printing plates to printing cylinders but have to be crosslinked in the preparation process following the coating operation. Moreover, these PSAs, owing to the multiplicity of ester groups and the resulting polarity, have a tendency toward high peel increase.
As a result the printing plates can be removed only with very high application of force. Moreover, the PSA must exhibit a certain hardness, so that, after bonding to the printing cylinder, the printing plates do not tend toward edge lifting over a prolonged time period.
There is therefore a need for a pressure-sensitive acrylate adhesive which is suitable for bonding printing plates and which meets the abovementioned requirements.